Thrombin is a trypsin-like serine protease having the activity essential to maintenance and development of life such as formation of hemostatic thrombus or curing of wound. Thrombin includes meizothrombin, α-thrombin, β-thrombin and γ-thrombin, among which α-thrombin is most important from the physiological point of view. Prothrombin, a precursor of thrombin, is biosynthetically produced in the hepatocytes in a vitamin K dependent manner and its blood level is in a range of 100 to 150 μg/ml. A vitamin K dependent coagulation factor has a Gla containing region (Gla domain) at the N-terminal and binds to a phospholipid via Ca2+ ion. It is known that, upon binding of the Gla domain to Ca2+ ion, a high dimensional overall structure of the protein is altered to thereby exert a key function as interacting with cofactors.
Prothrombin undergoes activation by FXa-FVa complex on the phospholipids of the cellular membrane wherein Arg320-Ile321 bonding in prothrombin is cleaved through restricted cleavage to form meizothrombin having the Gal domain and Kringle domain. Subsequently, Arg271-Thr272 bonding is cleaved through restricted cleavage to form α-thrombin, which is released from the cellular membrane and exhibits a variety of physiological activities by restrictedly cleaving a number of plasma proteins or various thrombin receptors on the cellular membrane. Alpha-thrombin is a two-chained molecule consisting of A chain and B chain. When B chain essential to the enzymatic activity undergoes autolysis, β-thrombin or γ-thrombin is produced to thereby lose an ability to activate fibrinogen or platelets.
Most of the formed thrombin participates locally in formation of larger thrombus by binding to fibrinous thrombus. Alpha-thrombin not only converts fibrinogen into fibrin but also activates FXIII to trigger cross-linkage of fibrin. Besides, α-thrombin may accelerate coagulation by activating FVIII and FV, a cofactor of FIX and FXa, respectively, to proceed coagulation. Thus, thrombin plays an important role in hemostasis and hence is a highly useful protein for use as a hemostatic.
On the other hand, thrombin changes its substrate specificity upon binding to thrombomodulin on the vascular endothelial cells to activate Protein C to promote anticoagulation. Thus, utilizing this enzymatic activity, thrombin is also useful as a process enzyme for preparing activated Protein C, an anticoagulant. Moreover, α-thrombin may potently coagulate and activate platelets and also exhibits a mitogenic activity. Alpha-thrombin, as displaying such a variety of physiological activities, has been used as a reagent in various fields of research and expected to still increase its utility in future.
With such functions and activities, thrombin has been widely used as a hemostatic, a process enzyme or a reagent for research. For example, for hemorrhage at the upper digestive tracts, thrombin derived from blood has been endoscopically spread or orally administered with successful hemostasis. Also, a fibrin paste used as a tissue adhesive comprises thrombin derived from blood together with fibrinogen and FXIII.